Do you have an alarm system? If the smoke detectors are wired into your alarm system, and something is wrong with the alarm (backup battery could be low, connection to the office could be shorting out, etc), the smoke detectors could go off.
And, hardwired smoke alarms are usually set so that if one is tripped, they all go off....at least ours are. So, if it is the optical sensor like others said, only one may need the cleaning, but they'll all go off.
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